1. Field of the Invention
The present invention relates to a sampling waveform digitizer and more particularly to a sampling waveform digitizer which is suitable for the production testing of high performance analog and data conversion components.
2. Description of the Prior Art
One of the most challenging requirements facing a supplier of high speed, precision data conversion components is the accurate and efficient measurement of dynamic performance parameters.
Important dynamic characteristics are usually determined via rather painstaking laboratory evaluation of a few randomly selected devices. Often, the procedure involves the use of several different fixtures, and requires skilled technicians to operate the instruments and record the results properly. The performance specifications are then published as "typical", or "guaranteed but not 100% tested", neither of which is very satisfactory from the customer's point of view.
Some measurements, such as settling time of a fast current output digital-to-analog converter (DAC), are traditionally so difficult to perform that the published specification is in fact only a "best guess" which the customer must verify by observing the device's apparent performance in his particular circuit.
Therefore, it is becoming increasingly desirable to perform these difficult dynamic measurements on a production basis as well as in the design laboratory. This requires that several different characteristics, including settling time, slew rate, bandwidth, time delay, and the like must be tested quickly, reliably, and with minimal socket changing or operator intervention.
These requirements became abundantly clear to the present inventors during the early development stages of a family of high speed data conversion components; namely, two fast-settling digital-to-analog converters and a high speed sample and hold amplifier. The digital-to-analog converters, both ECL and TTL input versions, were required to settle to 0.01% accuracy in 40 nanoseconds and the sample and hold amplifier was required to acquire a 10 volt signal to the same accuracy in approximately 250 nanoseconds. Because the most important design choices were those that affected the speed and accuracy performance, it was necessary to be able to measure the dynamic parameters reliably and verifiably. Also required was a technique suitable for medium to high volume production testing as well as one that customers could use for performance evaluation and incoming inspection.
Various other techniques or alternate approaches exist in the prior art, including (1) high speed clipping amplifiers with oscilloscope viewing of the output; (2) sampling oscilloscopes; (3) window comparator techniques; and (4) commercial waveform digitizers.
Conventional wideband oscilloscopes are suitable for measuring the dynamic characteristics to only 1 or 2% accuracy, at most. Precise settling time measurements cannot be made directly because the very large dynamic range of the signal overloads the oscilloscope amplifiers. Therefore, test circuits have been developed specifically to prevent overloading within the oscilloscope.
By clipping the test waveform with diodes or special "clipping amplifiers", it is possible to display the waveform on the most sensitive scale without severe overloading. However, the measurement accuracy still depends on several high speed, open loop amplifiers between the signal source and the display screen. The clipping circuit and amplifiers are themselves prone to thermal tails, ground loops and signal distortion.
Another prior art technique to prevent oscilloscope overloading is to sum the settling waveform with a step generator of the same amplitude but opposite polarity, so that the large signal excursions are cancelled out. This method requires that the settling time of the step generator itself be significantly shorter than that of the device under test, presenting a problem of test verification. If the settling characteristic of the step generator could be measured, the capability of making the original settling time measurement would already exist. Therefore, the step generator is assumed to settle well-based on theoretical circuit calculations rather than experimental verification.
Signal clipping and step generator techniques have been used successfully to measure current-mode DAC settling up to 12 bit resolution, but a high level of expertise in engineering art is required to implement them properly. Interpretation of the displayed waveform is subject to operator error and the test fixture is limited to settling time measurement alone. Evaluation of other parameters still requires a various assortment of fixtures and equipment hookup configurations within the laboratory.
Sampling oscilloscopes have a very high bandwidth and avoid the overload problem of conventional types, but the accuracy of the internal diode sampling bridge is limited to one or two millivolts. Also, there are numerous practical problems in attempting to drive the low input impedance.
An interesting method of testing settling time on a production basis exists in the prior art, and this system uses a window comparator with adjustable threshholds. Once the DC final value of the waveform is determined, a system of DACs sets the reference levels at the positive and negative limits of the error band. The test stimulus is then applied to the device under test (DUT), and the window comparator output is enabled after the allowable settling time has elapsed. If the DUT output then exceeds the error band, the comparator triggers a flip-flop to indicate a settling time failure.
The window comparator method is suitable for pass/fail production testing of moderately high speed waveforms. However, it does not lend itself to laboratory development or characterization work, because it gives no information above the actual shape of the waveform itself.
To meet the needs of both the development laboratory and the production floor, a system which permanently records the detailed waveshape is required. In other words, the ideal system is an accurate, high speed waveform digitizer.
Recognizing this, test equipment manufacturers have developed digitizers in various forms. Waveform recorders, transient recorders, and digital oscilloscopes are all designed to capture and store a set of time-amplitude points in digital form. Once the signal has been digitized, it is equally useful for production pass/fail decisions as for detailed engineering analysis.
While the digitizer concept is attractive, commercially available units do not yet offer the combination of bandwidth and resolution necessary for the type of high accuracy measurements under consideration herein, such as for the dynamic testing of high speed data conversion components and the like.
A sampling waveform digitizer is needed which is both highly accurate and generally useful for dynamic testing and characterization of various waveforms. It must be relatively inexpensive ad well-suited for both production and design engineering environments.
The present invention eliminates most of the deficiencies of the prior art and provides a sampling waveform digitizer for performing dynamic testing on high speed data conversion components including completely automated dynamic performance characterizations of sample and hold amplifiers and relatively fast digital-to-analog converters including accurate measurements of settling time. These have been implemented in the present invention and various system parameters can be measured including acquisition time, sample-to-hold settling time, aperture delay, glitch amplitude, sample-to-hold offset, feedthrough rejection, risetime, slew rate, and the like.